1. Field of the Invention
The present invention relates to a method of manufacturing an optical fiber in which a coating is formed on a glass optical fiber.
2. Description of the Related Art
An optical fiber is manufactured in a way described below. Firstly, a glass preform is heated in a furnace and melted at one end of the glass preform. Then the glass preform is drawn at the heated end to be formed into a glass optical fiber. Next, a resin is coated of the drawn glass fiber. Accordingly, the optical fiber is manufactured.
The coating of an optical fiber is generally formed in a way described below. Firstly, in order to have a resin applied on the glass optical fiber, the glass optical fiber is passed through a resin-coating die filled with a UV cured resin (hereinafter to be referred to as a resin when appropriate). Next, the glass optical fiber with the UV cured resin being applied is passed through a coating apparatus.
The coating apparatus is composed of a transparent tube and an ultraviolet source arranged in the periphery outer of the transparent tube. For example, a transparent tube is made of quartz glass. While passing through the transparent tube of the coating apparatus, a coated resin is cured by being irradiated with ultraviolet ray supplied from the UV source through the transparent tube of the coating apparatus, whereby the coating of optical fiber is formed.
In general, when a UV resin is cured in ambient air, that is in high oxygen concentrations atmosphere, the resin reacts with oxygen to cause insufficient curing, which leads to formation of a low-quality coating. As a solution to prevent this problem, a technology of irradiating a resin with UV in the presence of inert gas is disclosed (in Japanese Patent Application Laid-open No. 6-211545, for example).
Meanwhile, in the process of curing the resin, a portion of the resin is volatilized by the reaction heat generated in curing the resin and the heat generation caused by the absorption of light energy of the emitted light, and adheres to the inner surface of the transparent tube. Resin component adhering to the inner surface of the transparent tube will be altered by the UV irradiation to cause fogging in the transparent tube. The fogging reduces the amount of ultraviolet ray reaching the resin and causes insufficient curing, which also leads to formation of a low-quality coating.
As a solution of this problem, Japanese Patent Application Laid-open No. 2005-224689 discloses a technology of obtaining sufficient curing of the UV resin by supplying an inert gas to a running region of the glass optical fiber in the transparent tube while also supplying a gas partially containing oxygen to the periphery of the above-mentioned running region to form a two-laminar flow with the inert gas and the oxygen-contained gas on the surface of the transparent tube. With such two-laminar flow, fogging inside the transparent tube can be lessen.